1. Field of Invention
This invention relates in general to radio positioning/navigation systems. More specifically and in particular, the present invention, hereinafter described in accordance with the current best mode of practice, is such a radio positioning/navigation system that quickly extends global positioning system (GPS) signals into a shielded environment, or through a line-of-sight barrier via redundant RF carriers to overcome shielding such as ground overburden, jamming, and/or spoofing of the GPS signals by hostile or friendly forces, and combines this with ultra wideband to create a seamless, globally referenced positioning system.
These shielded environments where a globally referenced position is desirable include a structure, areas adjacent to the outer walls of buildings, a bunker deep underground, heavily forested areas, steep and narrow canyons, areas where GPS signals are being jammed or spoofed, etc.
2. Discussion of the Prior Art
A common need and requirement of our society, and for military and intelligence agencies around the world is to accurately track and record positions of personnel, aircraft, vehicles, geographical landmarks, supplies, buildings, and other objects. One method currently used to accomplish this goal uses radio positioning/navigation beacons and associated equipment. Radio positioning/navigation can be broadly defined as the use of radio waves to transmit information, which in turn can be received and utilized to determine position and to navigate. Some radio positioning/navigation systems either currently in use or under development, are Loran, Omega, and Global Navigation Satellite Systems (GPS) such as NAVSTAR, GLONASS (the Russian variant), and proposed European systems such as GALILEO and GRANAS. The radio positioning/navigation system quickly becoming the standard worldwide is the United States' NAVSTAR Global Positioning System (GPS). The NAVSTAR GPS system is capable of providing real-time, three-dimensional position and navigation data.
The NAVSTAR GPS beacon system presently consists of twenty-four orbiting satellites, spaced in six separate circular orbits, with each accommodating four satellites. Of these, twenty-one are normally operational and three serve as spares. Each NAVSTAR GPS satellite reappears above the same ground reference approximately every twenty-three hours and fifty-six minutes. The spacing of satellites is designed to maximize probability that earth users will always have at least four satellites in good geometrical view for navigational use. The basic method of position determination via radio positioning and navigation signals is derived from the concept of triangulation. The term triangulation used herein refers to the general process of determining distance, a.k.a. range, from the present position to multiple known beacons, and mathematically solving for the point in space which satisfies these conditions. As applied to GPS, the procedure requires calculation of signal travel time, which, when multiplied by the speed of light, renders distance.
A basic discussion of positioning/navigation as it relates to the NAVSTAR GPS is contained in a document entitled “GPS NAVSTAR—Global Positioning System, User's Overview” Reference Document YEE-82-009D, March 1991, prepared by ARINC Research Corporation. This document particularly describes the background of the NAVSTAR Global Positioning System, as well as technical descriptions, performance characteristics and actual user segments.
A process known as differential global positioning (DGPS) compensates for many of the errors which are common in radio positioning/navigation systems. An antenna at a known location receives line of sight (LOS) GPS signals and broadcasts a signal with current correction adjustments for each satellite which can be received by any differential receiver within its signal range.
Location accuracy via GPS is continually evolving. Standard GPS receivers can typically produce position estimates within +/−100 meter accuracy. Sub-meter position accuracy of location can be achieved using DGPS. Other techniques for improving accuracy are “Carrier-phase GPS”, “Wide Area Augmented GPS” (WAAS), and GPS Interferometry.
GPS relies on no visual, magnetic, or other point of reference which is particularly important in applications such as aviation and naval navigation that traverse polar regions where conventional magnetic navigational means are rendered less effective by local magnetic conditions. Magnetic deviations and anomalies common in standard radio positioning/navigation systems do not hinder GPS. In addition, GPS equipment is typically fabricated of standard, solid state electronic hardware, resulting in low cost, low maintenance systems, having few or no moving parts, and requiring no optics. GPS does not have the calibration, alignment, and maintenance requirements of conventional inertial measuring units, and is available 24 hours per day on a worldwide basis.
One critical limitation of these GPS systems is it requires the beacons to be in direct line-of-sight (LOS) of the receiver. In other words, if the GPS receiver is used in heavily forested areas, in steep and narrow canyons, within a structure, adjacent to the outer walls of buildings, or various other line-of-sight barriers (LSB) or other shielded environments where GPS signals may be jammed or spoofed, the receiver will be unable to obtain a good repeatable reading, or in many cases, any reading at all.
Operating within a line-of-sight barrier (LSB) places fundamental limitations on the performance of radio positioning/navigation systems. The existence of multi-path with different time delays, gives rise to complex, time-varying transmission channels. A direct line-of-site path between beacons and receiver seldom exists within a line-of-sight barrier (LSB), because of interference or reflection from natural or man-made objects, and one must rely on the signal arriving via multi-path. Signals can be received within a line-of-sight barrier (LSB), however to date, the equipment required to mitigate and correct for multi-path remains complex, sophisticated and the process is not repeatable. These multi-path problems, in effect, limit the practical commercial use of radio transmission of positioning/navigation data within a line-of-sight barrier (LSB).
Another dilemma associated with radio transmission of positioning/navigation data within a line-of-sight barrier (LSB) is a phenomenon known as the “near-far” problem. The near-far problem is due to simultaneous broadcasting of signals from multiple broadcast antennae. This problem arises because of the large variation of the user-to-broadcast antennae range. One advantage of using GPS signals is the average power being received from the GPS satellite beacons remains approximately constant due to the large distance of the satellite beacons from the GPS receiver(s). On the other hand in a local system, the broadcast antenna power from broadcast antennae varies a great deal, due to the inverse proportion to the square of the receiver's distance from the broadcast antennae, and can overwhelm other incoming signals.
Although there have been attempts to use radio positioning/navigation signals within a line-of-site barrier, to date the use of this technology is commercially impractical because of the problems described in the previous discussions. In departing radically from traditional RF techniques, ultra wideband (UWB) radio is a recent innovation in radio signal transmissions. Ultra wideband provides an innovative solution for local geo-positioning that overcomes multi-path and near-far problems.
Some line-of-sight barriers may be electronic. Recently, Fox News reported that Iraq may have obtained as many as 400 electronic “jammers” that could throw America's smart bombs off their programmed path if the U.S. goes to war. Jamming is an electronic warfare (EW) technique to limit the effectiveness of an opponents communications and/or detection equipment.
Communications jamming is usually aimed at radio signals to disrupt control of a battle. A transmitter, tuned to the same frequency as the opponents receiving equipment and with the same type of modulation, can with enough power, override any signal at the receiver. The most common types of this form of signal jamming are: Random Noise; Random Pulse; Stepped Tones; Wobbler; Random Keyed Modulated CW; Tone; Rotary; Pulse; Spark; Recorded Sounds; Gulls; and Sweep-through. All of these can be divided into two groups—obvious and subtle.
Obvious jamming is easy to detect, as it can be heard on the receiving equipment, and is usually some type of noise such as stepped tones (bagpipes), random-keyed code, pulses, erratically warbling tones, and recorded sounds. The purpose of this type of jamming is to block out reception of transmitted signals and to cause a nuisance to the receiving operator.
Subtle jamming is that in which no sound is heard on the receiving equipment. The radio does not receive incoming signals, yet everything seems superficially normal to the operator. These are often techical attacks on modern equipment, such as ‘SQUELCH capture’.
Radar jamming is the intentional emmission of radio frequency signals to interfere with the operation of a radar by saturating its receiver with false information. There are two types of radar jamming—noise jamming and deception jamming.
A noise jamming system is designed to delay or deny target detection. Noise jamming attempts to mask the presence of targets by substantially adding to the level of thermal noise received by the radar. Noise jamming usually employs high power signals tuned to the same frequency of the radar. The most common techniques include barrage, spot, swept spot, cover pulse, and modulated noise jamming and is usually employed by stand-off jamming (SOJ) assets or escort jamming assets.
Deception jamming systems (also called repeat jammers) are designed to offer false information to a radar to deny specific information on either bearing, range, velocity, or a combination of these. A deception jammer receives the radar signal, modifies it and retransmits the altered signal back to the radar.
AVIACONVERSIA is a Russian company that has made GPS/GLONASS jammers. These jammers have successfully jammed devices with P-code capability and are designed to jam both military and civilian frequencies. Also, these jammers have been designed to work with directional antennas in order to maintain safe areas for friendly forces to access GPS or GLONASS.
In a story in Computerworld, January 2003, entitled “Homemade GPS Jammers Raise Concerns”, government officials and communications experts assess the public safety and security implications of a newly posted online article that provides directions for making cheap devices that can jam Global Positioning System (GPS) signals.
Information that has appeared in the online hacker magazine Phrack, potentially puts GPS devices used for commercial navigation and military operations at risk, authorities said.
Although the article said the jammer is designed to work only against civil-use GPS signals broadcast on the frequency of 1035.42 MHz and not the military frequency of 1227.6 MHz, James Hasik, an Atlanta-based consultant and author of the book The Precision Revolution: GPS and the Future of Aerial Warfare, disagreed.
Hasik said that while the Phrack jammer is targeted at civil GPS signals, known as the C/A code, it could also threaten military systems, since “almost all military GPS receivers must first acquire the C/A signal” before locking onto the military signal, known as the P(Y) code.
Hasik said that GPS receivers are especially vulnerable to jamming because of low signal strength after traveling through space from GPS satellites orbiting 12,000 miles above the earth.
The U.S. Department of Defense, which faces the possibility of having its GPS-guided weapons come up against Russian-made GPS jammers in Iraq or elsewhere, has anti-jamming technology at its disposal. Still, Defense officials viewed the Phrack article with concern.
Experimental verification of GPS jam immunity has shown its vulnerability under simple intentional interference. The jammer is a CW transmitter of a pure sine-wave of a frequency, close to the carrier of satellite signals. As a result, the carrier and jamming signal beat and make impossible reception of data by the correlator.
GPS uses phase manipulated (PM) signals, which are thought to be highly resistant to interference. The PM signal is a sine-wave and in the predetermined time its phase is reversed step-like. The GPS receiver is based upon the correlator, where the multiplication of the already known waveform and the received signal portion is done, followed by integration over the low-pass band. Integration of the 1024 signal portions results in general reception of the single bit.
Therefore, GPS are highly susceptible to a jamming signal based on a CW sine wave. Thus, both the civilian and military channels can be jammed without knowledge of GPS codes.
The Federal Aviation Administration is developing a nationwide GPS-based precision landing system. The Coast Guard also operates a GPS-based maritime navigation system on both coasts, the Great Lakes, inland waterways and Hawaii. Bill Mosley, a spokesman for the Department of Transportation, the parent agency of the FAA and the Coast Guard, said his department is well aware of the threat posed by GPS jammers.
The DOT's John A. Volpe Transportation Systems Center, in Cambridge, Mass., prepared a report in August 2001 that said, “Some jamming devices/techniques are available on the Internet and proliferation will continue, because a single device that could disrupt military and civil operations worldwide would be attractive to malicious governments and groups.”
Although the GPS spread-spectrum signal offers some inherent antijam protection, an adversary who is determined to negate a GPS system need only generate a jamming signal with enough power and suitable temporal/spectral signature to deny the use of GPS throughout a given threat area. This vulnerability has been identified as a high priority within the Department of Defense (DOD), and numerous programs have been established to develop near-term solutions for today's potential threats and more extensive long-term solutions for projected future threats.
The first system developed to increase GPS antijam capability for users on the ground or in the air was the controlled reception pattern antenna. The underlying principle is fairly straightforward: received GPS signals are rather weak and cannot be detected or measured without a signal-correlation process; therefore, the processing algorithm assumes that any measurable energy above the ambient noise must be a jamming signal, and so it computes the necessary weights to null the source.
In addition, a controlled reception pattern antenna can only counter a limited number of jammers, as it eventually runs out of “degrees of freedom” or antijamming options when the number of spatially distributed jammers grows too great.
Various alternatives are being researched as part of the GPS Modernization and Navwar programs. The most obvious approach to increase antijam performance is to increase the transmitted power from the GPS satellites. Although the GPS Modernization program will increase satellite power, this approach alone will not provide the entire antijam performance that is required. It is therefore necessary to provide additional antijam capability from the user equipment. Basically, these user equipment techniques fall into two categories: those that reduce the jammer power while retaining or amplifying the GPS signal and those that increase the signal-to-noise ratio through advanced signal processing in the receiver (i.e., processing gain).
No one method is right for all circumstances because each application presents its own unique requirements and constraints. Moreover, a given technique may be effective against a particular class of threats, but may not necessarily address all threats. For example, an adaptive narrowband filter is effective against a jammer that has some repetitive or predictable signal structure, but is ineffective against a broadband noise jammer, whose signal cannot be predicted from previous samples. Likewise, spatial adaptive antenna arrays are effective against a limited number of broadband noise and structured signal jammers, but eventually run out of degrees of freedom as the number of jammers increases.
What is needed is a radio positioning/navigation system that overcomes the problems described above and provides repeatable, precise sub-centimeter positioning/navigation data for locating objects in direct line of sight (LOS) of GPS beacons, within line-of-sight of “bent path” GPS beacons, and within line-of-sight barriers (LSB) or other shielded environments. This system would be capable of overcoming shielding such as ground overburden, jamming, and/or spoofing of the GPS signals by hostile or friendly forces, and combines this with ultra wideband to create a seamless, globally referenced positioning system for a wide variety of applications.